This invention is directed to Stirling machines, both Stirling engines and Stirling coolers or heat pumps, that are drivingly coupled to electromagnetic transducers in the form of rotary electric motors and alternators for generating electrical power or using electrical power to drive a free-piston Stirling machine as a cooler or heat pump. The invention provides a manner of constructing them that reduces their mass and eliminates the friction of the motion converting drive between them and thereby eliminates the need for lubricants that contaminate the working gas of the Stirling machine. The invention improves the life, reliability and cost of the coupled Stirling machine and transducer while also avoiding the possibility of lubricant contamination of the working gas. The invention provides a higher ratio of power to mass thereby decreasing the size, weight, and volume of the coupled devices by using a rotary transducer and removes the need for lubricant by using a drive connection that has no friction and therefore does not require lubricant. The invention has the advantages of rotary alternator/motors without the disadvantage of lubricants that contaminate the working gas. The invention is particularly suited for alpha configuration Stirling machines but can also be applied to beta configured Stirling machines.
Stirling machines and particularly free-piston Stirling machines are very well known in the prior art and therefore the details of their operation are not described in this patent. The present invention uses the technology that is known in the art for the Stirling machine component of the invention except to the extent that structures and modifications are described. Similarly, the present invention uses the technology that is known in the art for rotary electromagnetic transducers except that the rotary electromagnetic transducers used with the invention are driven in a manner that differs from the conventional drive mode.
Free-piston Stirling machines are the only form of Stirling technology that have demonstrated a long reliable operational life. For practical implementations of a Stirling engine connected to an electromagnetic transducer for generating electrical power, some form of linear alternator has been used by all practitioners of the art.
FIG. 1 and FIG. 2 illustrate examples of such prior art implementations of a Stirling machine drivingly coupled to a linear electromagnetic transducer. FIG. 1 illustrates a conventional beta configured free-piston Stirling engine or heat pump drivingly coupled to a linear alternator or motor. This configuration has been the traditional vehicle for free-piston implementations. The displacer 1 shuttles the gas between the expansion space 2 and the compression space 3 while the piston 4 compresses the gas when it is mostly in the compression space and expands it when it is mostly in the expansion space. A linear alternator or motor 5 takes the output or provides the input depending on whether the machine is an engine (prime mover) or a heat pump, respectively. Since the moving magnets 7 of the linear alternator or motor 5 are directly connected to the piston 4, the amplitudes of the piston 4 and the magnets 7 are identical. The peak velocity of the piston 4 and the magnets 7 are therefore also identical. Balance mass 6 is resonated at or close to the operating frequency in order to reduce residual vibrations. The beta configuration suffers a central difficulty in that it scales poorly to higher powers. At higher powers, above about 1 kW, the machine becomes excessively large. The alpha configuration is the preferred embodiment for higher powers but linear alternator/motors do not scale well to the higher powers and therefore lead to large bulky embodiments and/or the need for expensive materials such as cobalt bearing alloys for the magnetic flux paths.
In order to apply the free-piston concept to alpha configured Stirling machines, linear alternator/motors 124 may be added to each piston as in U.S. Pat. No. 7,171,811 by Berchowitz and Kwon or as shown by U.S. Pat. No. 7,134,279 by White et al. as illustrated in FIG. 2. However, linear alternators or motors have a large mass compared to rotating alternators or motors and are therefore difficult to apply to alpha configured Stirling machines particularly at higher power levels of several kilowatts or above. More specifically, “power density” and “specific power” are terms used for an engineering parameter that describes the ratio of average or maximum power to mass. Although it is desirable to have the largest practical power density (power per unit of mass), linear electromagnetic transducers have a lower power density than rotary electromagnetic transducers. Therefore it would be desirable to have a practical manner of coupling an alpha configured Stirling machine to a rotary electromagnetic transducer.
FIG. 2 shows a four-cylinder alpha machine configured with a free-piston drive using linear alternators on each piston according to U.S. Pat. No. 7,134,279. Because linear alternators are bulky, the piston axes are forced apart further than would be ideal from a thermodynamic and packaging viewpoint. FIG. 2 shows the piston assemblies 118 and linear alternator assemblies 124 for a four-cylinder alpha configured free-piston Stirling engine. This sketch is taken from U.S. Pat. No. 7,134,279 and is numbered accordingly. Additional gas springs 228 are provided. Crank driven alpha configured machines suffer from the requirement of oil lubrication, extremely tight seals that separate the working gas from the oil and the resulting wear leading to low operational life and poor reliability. Up to now, there have been no practical mechanisms for alpha machines that have the intrinsic reliability of the beta-type free-piston machine.
Most large power alpha Stirling machines have utilized crank mechanisms driving rotary alternators. However, the alpha machines also have huge problems with friction in their motion converting mechanisms that convert the reciprocating motion of their pistons to rotary motion. The motion converting mechanisms that they use require conventional lubrication by conventional petroleum based or other lubricants. They also consume a lot of power overcoming friction and the friction causes them to wear out too soon. These prior art machines have suffered severe difficulties in separating the Stirling cycle working gas (usually helium or hydrogen) from the oil lubricant needed to lubricate their motion converting mechanisms. The seals that separate the working fluid from the oil suffer from high friction leading to power losses and rapid wear-out. As is well established in the prior art, free-piston machines do not themselves require oil lubrication and therefore do not require oil separating seals. Ideally, if alpha configured machines are to become long lived and reliable, it is necessary to find a mechanism that provides all the advantages of oil-free operation that are currently enjoyed by the beta configured free-piston machines but without the poor specific powers (power densities) associated with linear alternators.
Therefore, it is a principal object and purpose of the invention to provide an alpha configured Stirling machine that is drivingly coupled to a rotary electromagnetic transducer in a manner that eliminates the practical barrier to combining them, which is their high friction and resulting need for lubricants, in order to provide a resulting coupled Stirling machine and electromagnetic transducer that has no requirement for oil lubrication, friction-free operation, little mechanism wear, and high power density.